The caustic soda is sealed and has not leaked, although there are fears it may do so, Xinhua news agency said.

Half the ship was submerged near a harbour about halfway along the Yangtze river in central China's Hubei province, Xinhua said, citing local authorities.

Labourers discovered water was leaking into the ship as they were loading the caustic soda onto the vessel, near Yidu city, on Friday morning.

Rescue work was being hampered by heavy fog, with little visibility as other vessels tried to reach the sinking ship, according to Xinhua.

Caustic soda is highly corrosive and large amounts of it are toxic for aquatic organisms, according to the webpage of the Dow chemical company. It can also damage humans' skin and eyes.

The Yangzte river has become increasingly polluted in recent years as it has become a dumping ground for industrial, farming and human waste.

A record 30.5 billion tonnes of such waste were dumped into the river last year, twice as much as two decades ago, Xinhua reported last month.

Many species have been driven to the brink of extinction in the river in recent years, including the white-fin dolphin and Yangtze river sturgeon.

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